As Scott Hanselman mentions I've had some experience with SVG UI's in my current project at Serveron.
While I think SVG is a really cool idea, and amazingly useful for interactive graphics (if done succinctly) we had nothing but trouble trying to craft SVG UI components, which was largely the focus of Don's talk. There were just too many problems with different scripting/DOM models (you can access Adobe's SVG DOM from IE, but you can't get to IE from Adobe, etc.) to make it a long term workable solution. In retrospect I would have suggested (not having written the SVG personally) moving all the UI controls into ASPX and keeping the SVG strictly for graphics display.
While I thought Don's approach to some of the individual controls was cool (I really liked the analog gages), I don't think that SVG, or mor importantly the infrastructure build around SVG at present, is in good enough shape to be building UI's on it.
Scott's correct in that we've essentially scrapped the SVG UI for a WinForms based "Smart Client". We orignally banked on SVG to give us cross platform interactive graphics, but as always, it's not that simple. Adobe's SVG control, which seems to be pretty much the only game in town, doesn't work in Netscape 6, which pretty much kills the whole cross-platform thing, even if we could get the scripting to work.